In general, in order to transmit data as a radio wave signal, it is absolutely necessary to convert baseband signals (low frequency signal containing near-DC components) into high frequency radio signals; that is, modulation is indispensable. Modulation is roughly classified into three types. One is frequency modulation (FM for analog modulation, FSK for digital modulation) in which the frequency of high frequency signal is varied in proportion to baseband signals. For example, in FM radio broadcasting, signal modulation is performed using this frequency modulation before signal transmission.
In frequency modulation, as illustrated in FIG. 1, frequency deviation is determined by the instantaneous peak value of baseband signal, so a limiter circuit is commonly used for amplitude limitation so that the amplitude of baseband signal is prevented from exceeding a given value (maximum frequency deviation). However, when the amplitude of baseband signal is limited by the limiter circuit, the output of the limiter contains harmonic components. Thus, in order to eliminate the harmonic components, there is used a low-pass filter having a precipitous cutoff characteristic (for example, refer to Patent Document 1).    Patent Document 1: Japanese Patent Laid-Open No. 8-149012
FIG. 2 is a view illustrating a configuration of a baseband processing circuit used in the conventional FM modulator. Referring to FIG. 2, an amplifier 101 amplifies an input signal. A pre-emphasis circuit 102 emphasizes the signals outputted from the amplifier 101 as the frequency at the frequency band to be converted becomes higher in order to transmit signals with the higher frequency preliminarily and emphatically modulated. A limiter 103 limits the amplitude of the signal outputted from, the pre-emphasis circuit 102 to a prescribed limit value. A low-pass filter (LPF) 104 eliminates the high frequency components of the signal outputted from the limiter 103 and thereby eliminates harmonics produced in the limiter 103.
However, the output signal, of the limiter 103 contains n-th (n=1, 2, . . . ) harmonics, and thus even when the LPF 104 is used, it is difficult to eliminate all these harmonics. Further, when the voltage-frequency conversion characteristic of the FM modulator is not linear, the high frequency components which is expected to have been eliminated, by the LPF 104 appear in the modulation output signal, thus spreading the spectrum. Further, when the LPF 104 having a precipitous cutoff characteristic is used, there occurs phase distortion, i.e., group delay in which delay time varies for each frequency of the input signal.
It is noted here that, as indicated by the dashed-dotted line of FIG. 1, when the amplitude of baseband signal is controlled small, the peak value is prevented from exceeding the maximum frequency deviation. In this case, there is no need to use the limiter which causes occurrence of harmonics. However, when the amplitude of baseband signal is reduced in this way over the entire frequency range thereof, the average sound level decreases, reducing the sound of radio broadcasting or the like to be heard. Thus, this method is not preferable.